For the latter part, antacids can be used both for duodenal and gastric ulcers. Whether it is antacids, H2 receptor antagonists, or proton pump inhibitors, the end result would be decreasing the acidity of gastric milieu.
As for the differences between gastric and duodenal ulcers, if you have access to it, Goljan's Rapid Review Pathology has an excellent table summarising it (3rd Edition, p. 330). In essence, while there are some minor differences (basal/maximal acid output would be normal/decreased in gastic ulcers, whereas it would be increased in duodenal ulcers), both the etiology (H. pylori for both, almost 100% for duodenal and about 80% gastric), pathogenesis (imbalance between protective factors and mucosal insults) and treatment approaches (elimination of H. pylori) are similar.
